Mode-locked multi-mode fiber laser pulse source

ABSTRACT

A laser utilizes a cavity design which allows the stable generation of high peak power pulses from mode-locked multi-mode fiber lasers, greatly extending the peak power limits of conventional mode-locked single-mode fiber lasers. Mode-locking may be induced by insertion of a saturable absorber into the cavity and by inserting one or more mode-filters to ensure the oscillation of the fundamental mode in the multi-mode fiber. The probability of damage of the absorber may be minimized by the insertion of an additional semiconductor optical power limiter into the cavity.

FIELD OF THE INVENTION

[0001] The present invention relates to the amplification of single modelight pulses in multi-mode fiber amplifiers, and more particularly tothe use of multi-mode amplifying fibers to increase peak pulse power ina mode-locked laser pulse source used for generating ultra-short opticalpulses.

BACKGROUND OF THE INVENTION

[0002] Background Relating to Optical Amplifiers

[0003] Single-mode rare-earth-doped optical fiber amplifiers have beenwidely used for over a decade to provide diffraction-limited opticalamplification of optical pulses. Because single mode fiber amplifiersgenerate very low noise levels, do not induce modal dispersion, and arecompatible with single mode fiber optic transmission lines, they havebeen used almost exclusively in telecommunication applications.

[0004] The amplification of high peak-power pulses in adiffraction-limited optical beam in single-mode optical fiber amplifiersis generally limited by the small fiber core size that needs to beemployed to ensure single-mode operation of the fiber. In general theonset of nonlinearities such as self-phase modulation lead to severepulse distortions once the integral of the power level present insidethe fiber with the propagation length exceeds a certain limiting value.For a constant peak power P inside the fiber, the tolerable amount ofself-phase modulation Φ_(n1) is given by${\Phi_{nl} = {\frac{2\pi \quad n_{2}{PL}}{\lambda \quad A} \leq 5}},$

[0005] where A is the area of the fundamental mode in the fiber, λ isthe operation wavelength, L is the fiber length and n₂=3.2×10⁻²⁰ m²/W isthe nonlinear refractive index in silica optical fibers.

[0006] As an alternative to single-mode amplifiers, amplification inmulti-mode optical fibers has been considered. However, in general,amplification experiments in multi-mode optical fibers have led tonon-diffraction-limited outputs as well as unacceptable pulse broadeningdue to modal dispersion, since the launch conditions into the multi-modeoptical fiber and mode-coupling in the multi-mode fiber have not beencontrolled.

[0007] Amplified spontaneous emission in a multi-mode fiber has beenreduced by selectively exciting active ions close to the center of thefiber core or by confining the active ions to the center of the fibercore. U.S. Pat. No. 5,187,759, hereby incorporated herein by reference.Since the overlap of the low-order modes in a multi-mode optical fiberis highest with the active ions close to the center of the fiber core,any amplified spontaneous emission will then also be predominantlygenerated in low-order modes of the multi-mode fiber. As a result, thetotal amount of amplified spontaneous emission can be reduced in themulti-mode fiber, since no amplified spontaneous emission is generatedin high-order modes.

[0008] As an alternative for obtaining high-power pulses, chirped pulseamplification with chirped fiber Bragg gratings has been employed. Oneof the limitations of this technique is the relative complexity of theset-up.

[0009] More recently, the amplification of pulses to peak powers higherthan 10 KW has been achieved in multi-mode fiber amplifiers. See U.S.Pat. No. 5,818,630, entitled Single-Mode Amplifiers and CompressorsBased on Multi-Mode Fibers, assigned to the assignee of the presentinvention, and hereby incorporated herein by reference. As describedtherein, the peak power limit inherent in single-mode optical fiberamplifiers is avoided by employing the increased area occupied by thefundamental mode within multi-mode fibers. This increased area permitsan increase in the energy storage potential of the optical fiberamplifier, allowing higher pulse energies before the onset ofundesirable nonlinearities and gain saturation. To accomplish this, thatapplication describes the advantages of concentration of the gain mediumin the center of the multi-mode fiber so that the fundamental mode ispreferentially amplified. This gain-confinement is utilized to stabilizethe fundamental mode in a fiber with a large cross section by gainguiding.

[0010] Additionally, that reference describes the writing of chirpedfiber Bragg gratings onto multi-mode fibers with reduced mode-couplingto increase the power limits for linear pulse compression of high-poweroptical pulses. In that system, double-clad multi-mode fiber amplifiersare pumped with relatively large-area high-power semiconductor lasers.Further, the fundamental mode in the multi-mode fibers is excited byemploying efficient mode-filters. By further using multi-mode fiberswith low mode-coupling, the propagation of the fundamental mode inmulti-mode amplifiers over lengths of several meters can be ensured,allowing the amplification of high-power optical pulses in dopedmulti-mode fiber amplifiers with core diameters of several tens ofmicrons, while still providing a diffraction limited output beam. Thatsystem additionally employed cladding pumping by broad area diode arraylasers to conveniently excite multi-mode fiber amplifiers.

[0011] Background Relating to Modelocked Lasers

[0012] Both actively modelocked lasers and passively modelocked lasersare well known in the laser art. For example, compact modelocked lasershave been formed as ultrashort pulse sources using single-moderare-earth-doped fibers. One particularly useful fiber pulse source isbased on Kerr-type passive modelocking. Such pulse sources have beenassembled using widely available standard fiber components to providepulses at the bandwidth limit of rare-earth fiber lasers with GigaHertzrepetition rates.

[0013] Semiconductor saturable absorbers have recently foundapplications in the field of passively modelocked, ultrashort pulselasers. These devices are attractive since they are compact,inexpensive, and can be tailored to a wide range of laser wavelengthsand pulsewidths. Quantum well and bulk semiconductor saturable absorbershave also been used to modelock color center lasers

[0014] A saturable absorber has an intensity-dependent loss l. Thesingle pass loss of a signal of intensity I through a saturable absorberof thickness d may be expressed as

l=1−exp (−αd)

[0015] in which α is the intensity dependent absorption coefficientgiven by:

α(I)=α₀/(1+I/I _(SAT))

[0016] Here α₀ is the small signal absorption coefficient, which dependsupon the material in question. I_(SAT) is the saturation intensity,which is inversely proportional to the lifetime (τ_(A)) of the absorbingspecies within the saturable absorber. Thus, saturable absorbers exhibitless loss at higher intensity.

[0017] Because the loss of a saturable absorber is intensity dependent,the pulse width of the laser pulses is shortened as they pass throughthe saturable absorber. How rapidly the pulse width of the laser pulsesis shortened is proportional to |dq₀/dI|, in which q₀ is the nonlinearloss:

q ₀ =l(I)−l(I=0)

[0018] l(I=0) is a constant (=1−exp (−αd)) and is known as the insertionloss. As defined herein, the nonlinear loss q₀ of a saturable absorberdecreases (becomes more negative) with increasing intensity I. |dq₀/dI|stays essentially constant until I approaches I_(SAT), becomingessentially zero in the bleaching regime, i.e., when I>>I_(SAT).

[0019] For a saturable absorber to function satisfactorily as amodelocking element, it should have a lifetime (i.e., the lifetime ofthe upper state of the absorbing species), insertion loss l(I=0), andnonlinear loss q₀ appropriate to the laser. Ideally, the insertion lossshould be low to enhance the laser's efficiency, whereas the lifetimeand the nonlinear loss q₀ should permit self-starting and stable cwmodelocking. The saturable absorber's characteristics, as well as lasercavity parameters such as output coupling fraction, residual loss, andlifetime of the gain medium, all play a role in the evolution of a laserfrom startup to modelocking.

[0020] As with single-mode fiber amplifiers, the peak-power of pulsesfrom mode-locked single-mode lasers has been limited by the small fibercore size that has been employed to ensure single-mode operation of thefiber. In addition, in mode-locked single-mode fiber lasers, theroundtrip nonlinear phase delay also needs to be limited to around π toprevent the generation of pulses with a very large temporally extendedbackground, generally referred to as a pedestal. For a standardmode-locked single-mode erbium fiber laser operating at 1.55 μm with acore diameter of 10 μm and a round-trip cavity length of 2 m,corresponding to a pulse repetition rate of 50 MHz, the maximumoscillating peak power is thus about 1 KW.

[0021] The long-term operation of mode-locked single-mode fiber lasersis conveniently ensured by employing an environmentally stable cavity asdescribed in U.S. Pat. No. 5,689,519, entitled Environmentally StablePassively Modelocked Fiber Laser Pulse Source, assigned to the assigneeof the present invention, and hereby incorporated herein by reference.The laser described in this reference minimizes environmentally inducedfluctuations in the polarization state at the output of the single-modefiber. In the described embodiments, this is accomplished by including apair of Faraday rotators at opposite ends of the laser cavity tocompensate for linear phase drifts between the polarization eigenmodesof the fiber.

[0022] Recently the reliability of high-power single-mode fiber laserspassively mode-locked by saturable absorbers has been greatly improvedby implementing nonlinear power limiters by insertion of appropriatesemiconductor two-photon absorbers into the cavity, which minimizes thepeak power of the damaging Q-switched pulses often observed in thestart-up of mode-locking and in the presence of misalignments of thecavity. See U.S. patent application Ser. No. 09/149,369, filed on Sep.8, 1998, entitled Resonant Fabry-Perot Semiconductor Saturable Absorbersand Two-Photon Absorption Power Limiters, assigned to the assignee ofthe present invention, and hereby incorporated herein by reference.

[0023] To increase the pulse energy available from mode-lockedsingle-mode fiber lasers the oscillation of chirped pulses inside thelaser cavity has been employed. M. Hofer et al., Opt. Lett., vol. 17,page 807-809. As a consequence the pulses are temporally extended,giving rise to a significant peak power reduction inside the fiberlaser. However, the pulses can be temporally compressed down toapproximately the bandwidth limit outside the laser cavity. Due to theresulting high peak power, bulk-optic dispersive delay lines have to beused for pulse compression. For neodymium fiber lasers, pulse widths ofthe order of 100 fs can be obtained.

[0024] The pulse energy from mode-locked single-mode fiber lasers hasalso been increased by employing chirped fiber gratings. The chirpedfiber gratings have a large amount of negative dispersion, broadeningthe pulses inside the cavity dispersively, which therefore reduces theirpeak power and also leads to the oscillation of high-energy pulsesinside the single-mode fiber lasers.

[0025] See U.S. Pat. No. 5,450,427, entitled Technique for theGeneration of Optical Pulses in Mode-Locked Lasers by Dispersive Controlof the Oscillation Pulse Width, and U.S. Pat. No. 5,627,848, entitledApparatus for Producing Femtosecond and Picosecond Pulses from FiberLasers Cladding Pumped with Broad Area Diode Laser Arrays, both of whichare assigned to the assignee of the present invention and herebyincorporated herein by reference. In these systems, the generated pulsesare bandwidth-limited, though the typical oscillating pulse widths areof the order of a few ps.

[0026] However, though the dispersive broadening of the pulse widthoscillating inside a single-mode fiber laser cavity does increase theoscillating pulse energy compared to a ‘standard’ soliton fiber laser,it does not increase the oscillating peak power. The maximum peak powergenerated with these systems directly from the fiber laser is stilllimited to around 1 KW.

[0027] Another highly integratable method for increasing the peak powerof mode-locked lasers is based on using chirped periodically poledLiNbO3 (chirped PPLN). Chirped PPLN permits simultaneous pulsecompression and frequency doubling of an optically chirped pulse. SeeU.S. patent application Ser. No. 08/845,410, filed on Apr. 25, 1997,entitled Use of Aperiodic Quasi-Phase-Matched Gratings in UltrashortPulse Sources, assigned to the assignee of the present application, andhereby incorporated herein by reference. However, for chirped PPLN toproduce pulse compression from around 3 ps to 300 fs and frequencydoubling with high conversion efficiencies, generally peak powers of theorder of several KW are required. Such high peak powers are typicallyoutside the range of mode-locked single-mode erbium fiber lasers.

[0028] Broad area diode laser arrays have been used for pumping ofmode-locked single-mode fiber lasers, where very compact cavity designswere possible. The pump light was injected through a V-groove from theside of double-clad fiber, a technique typically referred to asside-pumping. However, such oscillator designs have also suffered frompeak power limitations due to the single-mode structure of theoscillator fiber.

[0029] It has also been suggested that a near diffraction-limited outputbeam can be obtained from a multi-mode fiber laser when keeping thefiber length shorter than 15 mm and selectively providing a maximumamount of feedback for the fundamental mode of the optical fiber.“Efficient laser operation with nearly diffraction-limited output from adiode-pumped heavily Nd-doped multimode fiber”, Optics Letters, Vol. 21,pp. 266-268 (1996) hereby incorporated herein by reference. In thistechnique, however, severe mode-coupling has been a problem, as theemployed multi-mode fibers typically support thousands of modes. Also,only an air-gap between the endface of the multi-mode fiber and a lasermirror has been suggested for mode-selection. Hence, only very poormodal discrimination has been obtained, resulting in poor beam quality.

[0030] While the operation of optical amplifiers, especially in thepresence of large seed signals, is not very sensitive to the presence ofspurious reflections, the stability of mode-locked lasers criticallydepends on the minimization of spurious reflections. Any strayreflections produce sub-cavities inside an oscillator and result ininjection signals for the cw operation of a laser cavity and thusprevent the onset of mode-locking. For solid-state FabryPerot cavities asuppression of intra-cavity reflections to a level <<1% (in intensity)is generally believed to be required to enable the onset ofmode-locking.

[0031] The intra-cavity reflections that are of concern in standardmode-locked lasers can be thought of as being conceptually equivalent tomode-coupling in multi-mode fibers. Any mode-coupling in multi-modefibers clearly also produces a sub-cavity with a cw injection signalproportional to the amount of mode-coupling. However, the suppression ofmode-coupling to a level of <<1% at any multi-mode fiber discontinuitiesis very difficult to achieve. Due to optical aberrations, even wellcorrected optics typically allow the excitation of the fundamental modein multi-mode fibers only with maximum efficiency of about 95%.Therefore to date, it has been considered that mode-locking of amulti-mode fiber is impossible and no stable operation of a mode-lockedmulti-mode fiber laser has yet been demonstrated.

SUMMARY OF THE INVENTION

[0032] This invention overcomes the foregoing difficulties associatedwith peak power limitations in modelocked lasers, and provides amode-locked multi-mode fiber laser.

[0033] This laser utilizes cavity designs which allow the stablegeneration of high peak power pulses from mode-locked multi-mode fiberlasers, greatly extending the peak power limits of conventionalmode-locked single-mode fiber lasers. Mode-locking may be induced byinsertion of a saturable absorber into the cavity and by inserting oneor more mode-filters to ensure the oscillation of the fundamental modein the multi-mode fiber. The probability of damage of the absorber maybe minimized by the insertion of an additional semiconductor opticalpower limiter into the cavity. The shortest pulses may also be generatedby taking advantage of nonlinear polarization evolution inside thefiber. The long-term stability of the cavity configuration is ensured byemploying an environmentally stable cavity. Pump light from a broad-areadiode laser may be delivered into the multi-mode fiber by employing acladding-pumping technique.

[0034] With this invention, a modelocked fiber laser may be constructedto obtain, for example, 360 fsec near-bandwidth-limited pulses with anaverage power of 300 mW at a repetition rate of 66.7 MHz. The peak powerof these exemplary pulses is estimated to be about 6 KW.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The following description of the preferred embodiments of theinvention references the appended drawings, in which like elements bearidentical reference numbers throughout.

[0036]FIG. 1 is a schematic illustration showing the construction of apreferred embodiment of the present invention which utilizes end-pumpingfor injecting pump light into the multi-mode fiber.

[0037]FIG. 2 is a graph showing the typical autocorrelation of pulsesgenerated by the invention of FIG. 1.

[0038]FIG. 3 is a graph showing the typical pulse spectrum generated bythe invention of FIG. 1.

[0039]FIG. 4 is a schematic illustration showing the construction of analternate preferred embodiment utilizing a side-pumping mechanism forinjecting pump light into the multi-mode fiber.

[0040]FIG. 5 is a schematic illustration of an alternative embodimentwhich uses a length of positive dispersion fiber to introduce chirpedpulses into the cavity.

[0041]FIG. 6 is a schematic illustration of an alternative embodimentwhich uses chirped fiber gratings with negative dispersion in the lasercavity to produce high-energy, near bandwidth-limited pulses.

[0042]FIGS. 7a and 7 b illustrate polarization-maintaining multi-modefiber cross sections which may be used to construct environmentallystable cavities in the absence of Faraday rotators.

[0043]FIG. 8 is a schematic illustration of an alternative embodimentwhich utilizes one of the fibers illustrated in FIGS. 7a and 7 b.

[0044]FIGS. 9a, 9 b and 9 c illustrate the manner is which thefundamental mode of the multi-mode fibers of the present invention maybe matched to the mode of a singe mode fiber. These include a bulk opticimaging system, as shown in FIG. 9a, a multi-mode to single-mode splice,as shown in FIG. 9b, and a tapered section of multi-mode fiber, asillustrated in FIG. 9c.

[0045]FIG. 10 is a schematic illustration of an alternative embodimentin which a fiber grating is used to predominantly reflect thefundamental mode of a multi-mode fiber.

[0046]FIG. 11 is a schematic illustration of an alternative embodimentin which active or active-passive modelocking is used to mode-lock themulti-mode laser.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0047]FIG. 1A illustrates the mode-locked laser cavity 11 of thisinvention which uses a length of multi-mode amplifying fiber 13 withinthe cavity to produce ultra-short, high-power optical pulses. As usedherein, “ultra-short” means a pulse width below 100 ps. The fiber 13, inthe example shown, is a 1.0 m length of non-birefringent Yb³⁺/Er³⁺-dopedmulti-mode fiber. Typically, a fiber is considered multi-mode when theV-value exceeds 2.41, i.e., when modes in addition to the fundamentalmode can propagate in the optical fiber. This fiber is coiled onto adrum with a diameter of 5 cm, though bend diameters as small as 1.5 cm,or even smaller, may be used without inhibiting mode-locking. Due to theEr³⁺ doping, the fiber core in this example has an absorption ofapproximately 40 dB/m at a wavelength of 1.53 μm. The Yb³⁺ co-dopingproduces an average absorption of 4.3 dB/m inside the cladding at awavelength of 980 nm. The fiber 13 has a numerical aperture of 0.20 anda core diameter of 16 μm. The outside diameter of the cladding of thefiber 13 is 200 μm. The fiber 13 is coated with a low-index polymerproducing a numerical aperture of 0.40 for the cladding. A 10 cm lengthof single-mode Corning Leaf fiber 15 is thermally tapered to produce acore diameter of approximately 14 μm to ensure an optimum operation as amode filter, and this length is fusion spliced onto a first end 17 ofthe multi-mode fiber 13.

[0048] In this exemplary embodiment, the cavity 11 is formed between afirst mirror 19 and a second mirror 21. It will be recognized that othercavity configurations for recirculating pulses are well known, and maybe used. In this example, the mirrors 19, 21 define an optical axis 23along which the cavity elements are aligned.

[0049] The cavity 11 further includes a pair of Faraday rotators 25, 27to compensate for linear phase drifts between the polarizationeigenmodes of the fiber, thereby assuring that the cavity remainsenvironmentally stable. As referenced herein, the phrase“environmentally stable” refers to a pulse source which is substantiallyimmune to a loss of pulse generation due to environmental influencessuch as temperature drifts and which is, at most, only slightlysensitive to pressure variations. The use of Faraday Rotators forassuring environmental stability is explained in more detail in U.S.Pat. No. 5,689,519 which has been incorporated by reference herein.

[0050] A polarization beam-splitter 29 on the axis 23 of the cavity 11ensures single-polarization operation of the cavity 11, and provides theoutput 30 from the cavity. A half-wave plate 31 and a quarter-wave plate33 are used to introduce linear phase delays within the cavity,providing polarization control to permit optimization of polarizationevolution within the cavity 11 for modelocking.

[0051] To induce mode-locking, the cavity 11 is formed as a Fabry-Perotcavity by including a saturable absorber 35 at the end of the cavityproximate the mirror 19. The saturable absorber 35 is preferably grownas a 0.75 μm thick layer of InGaAsP on one surface of a substrate. Theband-edge of the InGaAsP saturable absorber 39 is preferably chosen tobe 1.56 μm, the carrier life-time is typically 5 ps and the saturationenergy density is 100 MW/cm².

[0052] In this example, the substrate supporting the saturable absorber35 comprises high-quality anti-reflection-coated InP 37, with theanti-reflection-coated surface 39 facing the open end of the cavity 11.The InP substrate is transparent to single-photon absorption of thesignal light at 1.56 μm, however two photon absorption occurs. Thistwo-photon absorber 39 is used as a nonlinear power limiter to protectthe saturable absorber 35.

[0053] The mirror 19 in this exemplary embodiment is formed bydepositing a gold-film onto the surface of the InGaAsP saturableabsorber 35 opposite the two photon absorber 39. The combined structureof the saturable absorber 35, two photon absorber 37 and mirror 19provides a reflectivity of 50% at 1.56 μm. The surface of the gold-filmmirror 19 opposite the saturable absorber 35 is attached to a sapphirewindow 41 for heat-sinking the combined absorber/mirror assembly.

[0054] The laser beam from the fiber 15 is collimated by a lens 43 andrefocused, after rotation by the Faraday rotator 25, by a lens 45 ontothe anti-reflection-coated surface 39 of the two-photon absorber 37. Thespot size of the laser beam on the saturable absorber 35 may be adjustedby varying the position of the lens 45 or by using lenses with differentfocal lengths. Other focusing lenses 47 and 49 in the cavity 11 aid inbetter imaging the laser signal onto the multi-mode fiber 13.

[0055] Light from a Pump light source 51, such as a laser source, with awavelength near 980 nm and output power of 5 W, is directed through afiber bundle 57 with an outside diameter of 375 μm. This pump light isinjected into the end 53 of the multi-mode fiber 13 opposite thesingle-mode fiber 17. The pump light is coupled into the cavity 11 via apump signal injector 55, such as a dichroic beam-splitter for 980/1550nm. Lenses 47 and 48 are optimized for coupling of the pump power fromthe fiber bundle 57 into the cladding of the multi-mode fiber.

[0056] The M²-value of the beam at the output 30 of this exemplaryembodiment is typically approximately 1.2. Assuming the deterioration ofthe M²-value is mainly due to imperfect splicing between the multi-modefiber 13 and the single-mode mode-filter fiber 15, it can be estimatedthat the single-mode mode-filter fiber 15 excited the fundamental modeof the multi-mode fiber 13 with an efficiency of approximately 90%

[0057] Mode-locking may be obtained by optimizing the focussing of thelaser beam on the saturable absorber 35 and by optimizing theorientation of the intra-cavity waveplates 31, 33 to permit some degreeof nonlinear polarization evolution. However, the mode-locked operationof a multi-mode fiber laser system without nonlinear polarizationevolution can also be accomplished by minimizing the amount ofmode-mixing in the multi-mode fiber 13 and by an optimization of thesaturable absorber 35.

[0058] The pulses which are generated by the exemplary embodiment ofFIG. 1 will have a repetition rate of 66.7 MHz, with an average outputpower of 300 mW at a wavelength of 1.535 μm, giving a pulse energy of4.5 nJ. A typical autocorrelation of the pulses is shown in FIG. 2. Atypical FWHM pulse width of 360 fsec (assuming a sech² pulse shape) isgenerated. The corresponding pulse spectrum is shown in FIG. 3. Theautocorrelation width is within a factor of 1.5 of the bandwidth limitas calculated from the pulse spectrum, which indicates the relativelyhigh quality of the pulses.

[0059] Due to the multi-mode structure of the oscillator, the pulsespectrum is strongly modulated and therefore the autocorrelationdisplays a significant amount of energy in a pulse pedestal. It can beestimated that the amount of energy in the pedestal is about 50%, whichin turn gives a pulse peak power of 6 KW, about 6 times larger than whatis typically obtained with single-mode fibers at a similar pulserepetition rate.

[0060] Neglecting the amount of self-phase modulation in one passthrough the multi-mode fiber 13 and any self-phase modulation in themode-filter 15, and assuming a linear increase of pulse power in themulti-mode fiber 13 in the second pass, and assuming an effectivefundamental mode area in the multi-mode fiber 13 of 133 μm², thenonlinear phase delay in the multi-mode oscillator is calculated fromthe first equation above as Φ_(n1)=1.45π, which is close to the expectedmaximum typical nonlinear delay of passively mode-locked lasers.

[0061] The modulation on the obtained pulse spectrum as well as theamount of generated pedestal is dependent on the alignment of the mirror21. Generally, optimized mode-matching of the optical beam back into thefundamental mode of the multi-mode fiber leads to the best laserstability and a reduction in the amount of pedestal and pulse spectrummodulation. For this reason, optimized pulse quality can be obtained byimproving the splice between the single-mode filter fiber 15 and themulti-mode fiber 13. From simple overlap integrals it can be calculatedthat an optimum tapered section of Corning SMF-28 fiber 15 will lead toan excitation of the fundamental mode in the multi-mode fiber 13 with anefficiency of 99%. Thus any signal in higher-order modes can be reducedto about 1% in an optimized system.

[0062] An alternate embodiment of the invention is illustrated in FIG.4. As indicated by the identical elements and reference numbers, most ofthe cavity arrangement in this figure is identical to that shown inFIG. 1. This embodiment provides a highly integrated cavity 59 byemploying a side-pumping mechanism for injecting pump light into themulti-mode fiber 13. A pair of fiber couplers 61, 63, as are well knownin the art, inject light from a respective pair of fiber bundles 65 and67 into the cladding of the multi-mode fiber 13. The fiber bundles aresimilar to bundle 57 shown in FIG. 1, and convey light from a pair ofpump sources 69 and 71, respectively. Alternatively, the fiber bundles65, 67 and couplers 61, 63 may be replaced with V-groove light injectioninto the multi-mode fiber cladding in a manner well known in the art. Asaturable absorber 73 may comprise the elements 35, 37, 39 and 41 shownin FIG. 1, or may be of any other well known design, so long as itprovides a high damage threshold.

[0063] In another alternate embodiment of the invention, Illustrated inFIG. 5, the laser cavity 75 includes a positive dispersion element. AsWith FIG. 4, like reference numbers in FIG. 5 identify elementsdescribed in detail with reference to FIG. 1. In this embodiment, asection of single-mode positive dispersion fiber 77 is mounted betweenthe second mirror 21 and the lens 49. In a similar manner, a section ofpositive dispersion fiber could be spliced onto the end 53 of themulti-mode fiber 13, or the end of the single-mode mode-filter 15 facingthe lens 43. Positive dispersion fibers typically have a small corearea, and may limit the obtainable pulse energy from a laser. Theembodiment shown in FIG. 5 serves to reduce the peak power injected intothe positive dispersion fiber 77, and thus maximize the pulse energyoutput. This is accomplished by extracting, at the polarization beamsplitter 29, as much as 90-99% of the light energy.

[0064] In the embodiment of FIG. 5, the total dispersion inside thecavity may be adjusted to be zero to generate high-power pulses with alarger bandwidth. Alternatively, by adjusting the total cavitydispersion to be positive, chirped pulses with significantly increasedpulse energies may be generated by the laser.

[0065] The use of two single-mode mode-filter fibers 15, 77 is alsobeneficial in simplifying the alignment of the laser. Typically, tominimize modal speckle, broad bandwidth optical signals need to be usedfor aligning the mode-filter fibers with the multi-mode fiber. The useof two mode-filter fibers 15, 77 allows the use of amplified spontaneousemission signals generated directly in the multi-mode fiber for aniterative alignment of both mode-filters 15, 77.

[0066] The chirped pulses generated in the cavity 75 with overallpositive dispersion may be compressed down to approximately thebandwidth limit at the frequency doubled wavelength by employing chirpedperiodically poled LiNbO₃ 79 for sum-frequency generation, in a mannerwell known in the art. The chirped periodically poled LiNbO₃ 79 receivesthe cavity output from the polarization beam splitter 29 through anoptical isolator 81. In this case, due to the high power capabilities ofmulti-mode fiber oscillators, higher frequency-doubling conversionefficiencies occur compared to those experienced with single-mode fiberoscillators. Alternatively, bulk-optics dispersion compensating elementsmay be used in place of the chirped periodically poled LiNbO₃ 79 forcompressing the chirped pulses down to the bandwidth limit.

[0067] Generally, any nonlinear optical mixing technique such asfrequency doubling, Raman generation, four-wave mixing, etc. may be usedin place of the chirped periodically poled LiNbO₃ 79 to frequencyconvert the output of the multi-mode oscillator fiber 13 to a differentwavelength. Moreover, the conversion efficiency of these nonlinearoptical mixing processes is generally proportional to the lightintensity or light intensity squared. Thus, the small residual pedestalpresent in a multi-mode oscillator would be converted with greatlyreduced efficiency compared to the central main pulse and hence muchhigher quality pulses may be obtained.

[0068] As shown in the alternate embodiment of FIG. 6, very high-energyoptical pulses may also be obtained by inserting a chirped fiber gratingsuch as a Bragg grating 83, with negative dispersion, into the cavity85. Such a system typically produces ps length, high-energy,approximately bandwidth-limited pulses. Due to the multi-mode fiberused, much greater peak powers compared to single-mode fiber oscillatorsare generated. Here the fiber grating 83 is inserted after thepolarization beam splitter 29 to obtain an environmentally-stable cavityeven in the presence of nonpolarization maintaining multi-mode fiber 13.

[0069] In each of the embodiments of this invention, it is advantageousto minimize saturation of the multi-mode fiber amplifier 13 by amplifiedspontaneous emission generated in higher-order modes. This may beaccomplished by confining the rare-earth doping centrally within afraction of the core diameter.

[0070] Polarization-maintaining multi-mode optical fiber may beconstructed by using an elliptical fiber core or by attachingstress-producing regions to the multi-mode fiber cladding. Examples ofsuch fiber cross-sections are shown in FIGS. 7a and 7 b, respectively.Polarization-maintaining multi-mode fiber allows the construction ofenvironmentally stable cavities in the absence of Faraday rotators. Anexample of such a design is shown in FIG. 8 in this case, the output ofthe cavity 87 is provided by using a partially-reflecting mirror 89 atone end of the cavity 87, in a manner well known in this art.

[0071] To ensure optimum matching of the fundamental mode of themulti-mode fiber 13 to the mode of the single-mode mode-filter fiber 15in each of the embodiments of this invention, either a bulk opticimaging system, a splice between the multi-mode fiber 13 and thesingle-mode fiber 15, or a tapered section of the multi-mode fiber 13may be used. For example, the multi-mode fiber 13, either in the formshown in one for FIG. 7a and FIG. 7b or in a non-polarizationmaintaining form may be tapered to an outside diameter of 70 μm. Thisproduces an inside core diameter of 5.6 μm and ensures single modeoperation of the multi-mode fiber at the tapered end. By furtheremploying an adiabatic taper, the single-mode of the multi-mode fibermay be excited with nearly 100% efficiency. A graphic representation ofthe three discussed methods for excitation of the fundamental mode in anmulti-mode fiber 13 with a single-mode fiber mode-filter 15 is shown inFIGS. 9a, 9 b and 9 c, respectively. The implementation in a cavitydesign is not shown separately, but the splice between the single-modefiber 15 and the multi-mode fiber 15 shown in any of the disclosedembodiments may be constructed with any of the three alternatives shownin these figures.

[0072]FIG. 10 shows an additional embodiment of the invention. Here,instead of single-mode mode-filter fibers 15 as used in the previousembodiments, fiber gratings such as a Bragg grating directly writteninto the multi-mode fiber 13 is used to predominantly reflect thefundamental mode of the multi-mode fiber 13. Light from the pump 51 isinjected through the fiber grating 97 to facilitate a particularlysimple cavity design 99. Both chirped fiber gratings 97 as well asunchirped gratings can be implemented. Narrow bandwidth (chirped orunchirped) gratings favor the oscillation of pulses with a bandwidthsmaller than the grating bandwidth.

[0073] Finally, instead of passive mode-locking, active mode-locking oractive-passive mode-locking techniques may be used to mode-lockmulti-mode fibers. For example, an active-passive mode-locked systemcould comprise an optical frequency or amplitude modulator (as theactive mode-locking mechanism) in conjunction with nonlinearpolarization evolution (as the passive mode-locking mechanism) toproduce short optical pulses at a fixed repetition rate without asaturable absorber. A diagram of a mode-locked multi-mode fiber 13 witha optical mode-locking mechanism 101 is shown in FIG. 11. Also shown isan optical filter 103, which can be used to enhance the performance ofthe mode-locked laser 105.

[0074] Generally, the cavity designs described herein are exemplary ofthe preferred embodiments of this invention. Other variations areobvious from the previous discussions. In particular, opticalmodulators, optical filters, saturable absorbers and a polarizationcontrol elements are conveniently inserted at either cavity end.Equally, output coupling can be extracted at an optical mirror, apolarization beam splitter or also from an optical fiber couplerattached to the single-mode fiber filter 15. The pump power may also becoupled into the multi-mode fiber 13 from either end of the multi-modefiber 13 or through the side of the multi-mode fiber 13 in any of thecavity configurations discussed. Equally, all the discussed cavities maybe operated with any amount of dispersion. Chirped and unchirpedgratings may be implemented at either cavity end to act as opticalfilters and also to modify the dispersion characteristics of the cavity.

What is claimed is:
 1. A laser for generating ultra-short opticalpulses, comprising: a cavity which repeatedly passes light energy alonga cavity axis; a length of multi-mode optical fiber doped with a gainmedium and positioned along said cavity axis; a pump for exciting saidgain medium; a mode locking mechanism positioned on said cavity axis;and an optical guide positioned on said cavity axis which confines thelight amplified by said multi-mode optical fiber to preferentially thefundamental mode of said multi-mode optical fiber.
 2. A laser forgenerating ultra-short optical pulses as defined in claim 1 wherein saidmode locking mechanism comprises a passive mode locking element.
 3. Alaser for generating ultra-short optical pulses as defined in claim 2wherein said passive mode locking element comprises a saturableabsorber.
 4. A laser for generating ultra-short optical pulses asdefined in claim 3 wherein said saturable absorber comprises InGaAsP. 5.A laser for generating ultra-short optical pulses as defined in claim 3additionally comprising a power limiter for protecting said saturableabsorber.
 6. A laser for generating ultra-short optical pulses asdefined in claim 5 wherein said power limiter comprises a two photonabsorber.
 7. A laser for generating ultra-short optical pulses asdefined in claim 1 wherein said optical guide comprises a single-modemode-filter fiber on said cavity axis.
 8. A laser for generatingultra-short optical pulses as defined in claim 7 wherein saidsingle-mode mode-filter fiber is fusion spliced onto one end of saidmulti-mode optical fiber.
 9. A laser for generating ultra-short opticalpulses as defined in claim 8 wherein said multi-mode fiber is tapered atsaid fusion splice.
 10. A laser for generating ultra-short opticalpulses as defined in claim 8 wherein said single-mode mode-filter fiberis tapered at said fusion splice.
 11. A laser for generating ultra-shortoptical pulses as defined in claim 8 wherein both said single-modemode-filter fiber and said multi-mode fiber are tapered at said fusionsplice.
 12. A laster for generating ultra-short optical pulses asdefined in claim 1 wherein said pump is coupled to said multi-mode fiberalong said cavity axis.
 13. A laser for generating ultra-short opticalpulses as defined in claim 1 wherein said pump is coupled to the side ofsaid multi-mode fiber.
 14. A laser for generating ultra-short opticalpulses as defined in claim 13 additionally comprising an optical couplerfor coupling said pump to said multi-mode fiber.
 15. A laser forgenerating ultra-short optical pulses as defined in claim 13additionally comprising a v-groove on said multi-mode optical fiber forcoupling said pump to said multi-mode fiber.
 16. A laser for generatingultra-short optical pulses as defined in claim 1 additionally comprisinga polarization beam splitter for outputting said ultra-short opticalpulses from said laser.
 17. A laser for generating ultra-short opticalpulses as defined in claim 1 wherein said cavity comprises a pair ofreflectors at its opposite ends.
 18. A laser for generating ultra-shortoptical pulses as defined in claim 17 wherein one of said pair ofreflectors is partially reflecting and provides the output for saidcavity.
 19. A laser for generating ultra-short optical pulses as definedin claim 17 wherein said mode locking mechanism comprises a saturableabsorber, and wherein one of said reflectors is formed on a surface ofsaid saturable absorber.
 20. A laser for generating ultra-short opticalpulses as defined in claim 19 wherein said mode locking mechanismadditionally comprises a power limiter for protecting said saturableabsorber, and wherein said saturable absorber is formed on a surface ofsaid power limiter opposite said one of said reflectors.
 21. A laser forgenerating ultra-short optical pulses as defined in claim 20 whereinsaid power limiter comprises a two-photon absorber.
 22. A laser forgenerating ultra-short optical pulses as defined in claim 1 additionallycomprising a linear phase drift compensator on said cavity axis.
 23. Alaser for generating ultra-short optical pulses as defined in claim 22wherein said linear phase drift compensator comprises a Faraday rotator.24. A laser for generating ultra-short optical pulses as defined inclaim 23 wherein said linear phase drift compensator comprises a pair ofFaraday rotators.
 25. A laser for generating ultra-short optical pulsesas defined in claim 22 additionally comprising a linear polarizationtransformer on said cavity axis.
 26. A laser for generating ultra-shortoptical pulses as defined in claim 25 wherein said linear polarizationtransformer comprises a wave plate.
 27. A laser for generatingultra-short optical pulses as defined in claim 1 wherein said modelocking mechanism comprises an active mode locking element.
 28. A laserfor generating ultra-short optical pulses as defined in claim 27 whereinsaid active mode locking element comprises an optical amplitudemodulator.
 29. A laser for generating ultra-short optical pulses asdefined in claim 27 wherein said active mode locking element comprisesan optical frequency modulator.
 30. A laser for generating ultra-shortoptical pulses as defined in claim 1 wherein said ultra-short opticalpulses preferentially in the fundamental mode of said multi-mode opticalfiber have a pulse width below 500 psec.
 31. A laser for generatingultra-short optical pulses as defined in claim 1 additionally comprisingan environmental stabilizer on said cavity axis to assure that saidcavity remains environmentally stable.
 32. A laser for generatingultra-short optical pulses as defined in claim 31 wherein saidenvironmental stabilizer comprises a Faraday rotator.
 33. A laser forgenerating ultra-short optical pulses as defined in claim 32 whereinsaid environmental stabilizer comprises a pair of Faraday rotators. 34.A laser for generating ultra-short optical pulses as defined in claim 1wherein said optical guide comprises an optical fiber doped with anamplifying medium to provide gain guiding.
 35. A laser for generatingultra-short optical pulses as defined in claim 34 wherein saidamplifying medium is concentrated centrally within a fraction of thecore diameter of said optical fiber.
 36. A laser for generatingultra-short optical pulses as defined in claim 1 wherein said opticalguide comprises a single-mode optical fiber on said cavity axis.
 37. Alaser for generating ultra-short optical pulses as defined in claim 1wherein said optical guide comprises a mode-filter on said cavity axis.38. A laser for generating ultra-short optical pulses as defined inclaim 37 wherein said mode filter excites the fundamental mode of saidmulti-mode fiber.
 39. A laser for generating ultra-short optical pulsesas defined in claim 38 wherein said mode filter excites the fundamentalmode of said multi-mode fiber with an efficiency of at least 90%.
 40. Alaser for generating ultra-short optical pulses as defined in claim 1wherein said cavity additionally comprises a positive dispersionelement.
 41. A laser for generating ultra-short optical pulses asdefined in claim 40 wherein said positive dispersion element comprises alength of single-mode positive dispersion fiber positioned along saidcavity axis.
 42. A laser for generating ultra-short optical pulses asdefined in claim 41 additionally comprising an output coupler forlimiting the light energy at said single-mode positive dispersion fiberto less than 10% of the peak power in said cavity.
 43. A laser forgenerating ultra-short optical pulses as defined in claim 42additionally comprising a frequency converter for compressing pulsesgenerated by said cavity.
 44. A laser for generating ultra-short opticalpulses as defined in claim 43 wherein said frequency converter comprisesa frequency doubler.
 45. A laser for generating ultra-short opticalpulses as defined in claim 44 wherein said frequency doubler compriseschirped periodically poled LiNbO₃.
 46. A laser for generatingultra-short optical pulses as defined in claim 1 wherein said multi-modefiber includes a core, and wherein said gain medium in said multi-modeoptical fiber is concentrated centrally within the core of saidmulti-mode fiber.
 47. A laser for generating ultra-short optical pulsesas defined in claim 1 wherein said multi-mode optical fiber ispolarization-maintaining.
 48. A laser for generating ultra-short opticalpulses as defined in claim 47 wherein said polarization-maintainingmulti-mode fiber has an elliptical core.
 49. A laser for generatingultra-short optical pulses as defined in claim 47 wherein saidpolarization maintaining multi-mode fiber comprises stress-producingregions.
 50. A laser for generating ultra-short optical pulses asdefined in claim 1 wherein said cavity additionally comprises a fibergrating written onto said multi-mode fiber, said grating primarilyreflecting the fundamental mode of said multi-mode fiber.
 51. A methodof generating ultra-short pulses, comprising: providing a length ofoptical fiber doped with a gain medium; repeatedly passing signal lightthrough said length of optical fiber to produce said ultra-short pulses;and providing sufficient stored energy within said gain medium toamplify said pulses to a peak power above 1 KW.
 52. A method ofgenerating ultra-short pulses as defined in claim 51 additionallycomprising: environmentally stabilizing said optical fiber.
 53. A methodof generating ultra-short pulses as defined in claim 51 additionallycomprising modelocking said optical fiber.
 54. A method of generatingultra-short pulses as defined in claim 51 wherein said providing stepcomprises providing a multi-mode fiber doped with a gain medium.
 55. Amethod of generating ultra-short optical pulses, comprising: circulatinglight energy within a cavity; amplifying said light energy within saidcavity in a multi-mode fiber; and confining said light energy withinsaid cavity substantially to the fundamental mode of said multi-modefiber.
 56. A method of generating ultra-short optical pulses as definedin claim 55 additionally comprising mode locking said light energy. 57.A method of generating ultra-short optical pulses as defined in claim 55wherein said confining comprises mode filtering said light energy.
 58. Amode-locked laser for generating high power ultra-short optical pulses,comprising: A multi-mode optical fiber doped with gain material foramplifying optical energy; means for pumping said optical fiber; andmeans for confining the optical energy amplified by said multi-modeoptical fiber to substantially the fundamental mode of said multi-modeoptical fiber.